(actor and actress).
Usually an (m) word with an '-ier' ending will become '-ière' in the (f) form (eg le fermier, la fermière) and an (m) word with an '-ien' ending will become '-ienne' in the (f) form [much as with adjectives], eg le chirurgien, la chirurgienne (surgeon). If you've absorbed the basic rules for the m/f forms of adjectives, nouns follow pretty much the same patterns.
Some words, like 'pupil', are the same for both boys and girls ('l'élève' for both, though you would have 'un élève (m)' and 'une élève (f)').
..... and some words are absolutely definitely *only* masculine or *only* feminine.
For example "le professeur" translates "the teacher". So "Elle est le professeur" ("she is the teacher") is correct.
Equally unusual is "la personne" (always feminine). So "Il est la personne ..." ("he is the person .....") is also correct!
Not a great deal to remember, here. See Articles and Adjectives, for other parallels.